A key component in many copy machines and laser printers is a photoreceptor onto which a powdered "ink" or toner is deposited in a precise pattern. The pattern is generated by light exposure, and corresponds to the images or letters to be copied or printed. Paper is placed into contact with the photoreceptor, toner is transferred to the paper, and the "inked" paper is subjected to heat and pressure to fuse the toner onto the paper. The photoreceptor is then wiped clean of any remaining toner, and the process is repeated.
Among many possible configurations, an important commercial photoreceptor design is a multilayer structure wherein a metal drum is coated with an insulating polymer base layer, and an organic photoconductive (OPC) overcoat consisting of a charge generating layer and a charge transport layer. In the copying or laser printing process, deposition of the toner on the photoconductive drum is effected by creating a negatively or positively charged pattern on the drum to which oppositely charged particles of toner powder cling. This initial charged pattern is created by imparting a charge of the desired polarity to the entire drum. After electrostatic charging, the cylinder is carefully exposed to a pattern of light which causes charge carriers to be freed within the charge generating layer of the drum. These charge carriers, under the influence of the existing electric field, migrate through the charge transport layer, neutralize the deposited charge, thereby electrically discharging those portions. The remaining charged portions of the surface correspond to the pattern to which the toner is electrostatically attracted.
To produce clear and consistent images, the photoconductive drum must be able to consistently accept a sufficient level of charge, hold the charge for a sustained period of time in the dark, and discharge rapidly under controlled light exposure over repeated cycles. Devices presently exist which enable tests of certain charge build-up, charge retention, and discharge characteristics to be performed. However, these devices are not suitable for measuring other quality characteristics during the manufacture of new drums, or in the recycling of used drums. These characteristics include the cleanliness of the underlying metal drum surface, the uniformity of thickness of the polymer base and charge generation layers, the presence of layer defects, and various measures of electrophotographic performance across the entire drum surface.
While generalized techniques for nondestructive detection and characterization of flaws in materials, such as those described in U.S. Pat. No. 4,443,764 to Suh et al. are known, there is no automated test apparatus or series of tests that is capable of evaluating the full range of photoconductive drum characteristics that bear on print quality. Such tests and an easily operated apparatus for performing them would be important to quality control during manufacture and refurbishment of products having photoconductive drums.